Atelectasis simply means collapse of the lung parenchyma. This includes failure of expansion of the lung parenchyma( Congenital) and a normally inflated lung parenchyma getting collapsed(Acquired).
There are 3 types of Acquired Atelectasis-
1. Resorption Atelectasis
Basically an obstruction in the airway causes collapse of the airway distally. The mediastinum shifts towards the side of the collapsed lung.
2. Compression Atelectasis
The lung collapses due to the compressing force which may be due to hydro thorax/hemothorax etc. The mediastinum shifts to the opposite side of the collapsed lung.
3. Contraction Atelectasis
Due to chronic interstitial fibrosis which makes the lung smaller. The only type of Atelectasis which is irreversible.